1. Field of the Invention
The present invention relates generally to molecularly imprinted materials, and, more particularly, to a molecularly imprinted polymer material including selective binding sites for cations and anions, for example, ferric, phosphate, or nitrate ions.
2. Description of the Related Art
The concept of molecularly imprinting molecules may be traced to suppositions about the operation of the human immune system made by Stuart Mudd, circa 1930, and Linus Pauling, circa 1940. Mudd proposed the idea of complementary structures, by which a specific antibody attaches to a specific target or antigen because the shape of the antibody provides a cavity for receiving the shape of the antigen. The xe2x80x9clock and keyxe2x80x9d analogy used in explaining the action of enzymes is similarly explained, where enzymes form a xe2x80x9clockxe2x80x9d for a particular chemical xe2x80x9ckeyxe2x80x9d. Pauling postulated on how an otherwise nonspecific antibody molecule could be reorganized into a specific binding molecule. He reasoned that shape specificity was obtained when the body assembled a new protein complement, i.e., antibody, by using a target antigen as a template in arranging the complementary shape of an antibody. A nonspecific molecule can thus be shaped to the contours of a specific target. When the target is removed, the shape of the target is retained, which provides an antibody with a propensity to rebind the antigen. This process is known as molecular imprinting.
Molecular imprinting is used to create specific recognition sites in substrate materials, for example, polymeric organic materials. Known molecular imprinting techniques involve crosslinking materials in the presence of a functional monomer or mixture of monomers. Reactive or coordination sites on a target molecule or complex interact with a complementary site on a functional monomer during the polymerization process, either covalently or by other interactions such as ionic, hydrophobic or hydrogen bonding. Upon removal of the target molecule from the substrate, a xe2x80x9ccavityxe2x80x9d or recognition site is formed for receiving a similarly shaped molecule.
Synthetic production of molecularly imprinted polymers with selective binding sites for a specific target cation is achieved by providing polymers with cavities lined with complexing groups or xe2x80x9cligandsxe2x80x9d arranged to match the charge, coordination number, coordination geometry, and size of the target cation. Molecularly imprinted polymers with selective binding sites for anions are made in a similar manner to cations, but typically employ a trapped metal ion that has a high affinity for the target anion. Cavity-containing polymers may be produced using a specific ion as a template around which monomeric complexing ligands will be self-assembled and polymerized. Complexing ligands contain functional groups known to form stable complexes with the specific target ion and less stable complexes with other ions.
A method of molecular imprinting referred to as solution polymerization results in the formation of imprinted sites that are completely encased within the polymer. To access those sites, the polymer must be ground to produce particles that have exposed sites. The grinding process, however, produces irregularly shaped particles and also damages the sites by adversely affecting selectivity and activity. As an alternative method to increase accessibility to the imprinted sites is by using porogens, which are typically inert solvents, which when removed, create pores to allow access to the created binding sites. Removal of the porogen solvent adversely affects the structural integrity of the polymer, leading to deformation of the sites and loss in specificity and activity.
Molecular imprinting is useful in a variety of applications. For example, the ability to remove a specific component from its environment applies to both environmental and medical fields.
According to the Environmental Protection Agency, approximately 40 percent of the waterways in the United States still do not meet water quality goals and about half of the 2000 major watersheds have water quality problems. Phosphorus and nitrogen are major pollutants that enter our waterways as runoff from sewage plants and farmland, posing a clear threat to drinking water and aquatic life. As the nitrates increase in the environment, they act as plant nutrients, and cause an increase in plant growth. As plant material dies and decomposes, dissolved oxygen levels decrease. An increase in nitrates may be followed by an increase in phosphates. As phosphates increase and the growth of aquatic plants is encouraged, algal blooms can occur. With an increase in algal growth and decomposition, the dissolved oxygen levels further decrease, causing the death of fish and disruption to the ecosystem. Likewise, excess nitrate levels in drinking water are dangerous and can result in health risks, such as xe2x80x9cblue babyxe2x80x9d syndrome, which is a diminished capacity of the blood in transporting and transferring oxygen.
In the medical field, there are no known methods for removing excess iron from the blood of patients with extremely high iron levels, such as those suffering from Iron Overload Disease, also known as Hemochromatosis, and iron overdose, without attendant problems. Presently known treatments include replacement of the fatally iron rich blood by transfusion or chelation therapy, by which important electrolytes are lost and additional problems are introduced.
There remains a need for an effective and easy to use method and apparatus for removing iron, nitrate, or phosphate ions from an environment.
Briefly described, a method for making a molecularly imprinted polymer membrane according to an aspect of the invention is provided. The membrane includes a selective binding site for phosphate anions. The method for making the membrane includes the steps of copolymerizing a solution comprising a matrix monomer, a cross-linking agent, a lanthanide-phosphate anion imprinting complex, a permeability agent, and an initiator for the polymerization mixture, so that a polymer membrane is formed, and, removing the phosphate anions and the permeability agent from the polymer membrane so that cavities and channels for receiving phosphate anions are formed in the membrane. The cavities are in communication with the exterior surface of the membrane by means of the channels. The imprint monomer is preferably styrene, the cross-linking agent is preferably divinylbenzene, the initiator is preferably azobisisobutyronitrile, and the permeability agent is preferably polyester. The lanthanide-phosphate anion imprinting complex is preferably Ln (lanthanide) (III) tris(vinylbenzoylacetonate) (acryamidophenanthroline)biphosphate. Polyester and phosphate anions are removed by contacting the polymer membrane with an acidic solution.
A method for making a molecularly imprinted polymer membrane including a selective binding site for nitrate anions according to an aspect of the invention is also provided. The method includes the steps of copolymerizing a solution comprising a matrix monomer, a cross-linking agent, a nickel-nitrate anion imprinting complex, a permeability agent, and an initiator for the polymerization mixture, so that a polymer membrane is formed, and removing nitrate anions and the permeability agent from said polymer membrane so that cavities and channels for receiving nitrate anions are formed in the membrane. The cavities are in communication with the exterior surface of the membrane by means of the channels. The imprint monomer is preferably styrene, the cross-linking agent is preferably divinylbenzene, the initiator is preferably azobisisobutyronitrile, and the permeability agent is preferably polyester. The nickel-nitrate anion imprinting complex is preferably nickel (or Ni) (II) bis-(acrylamidophenanthroline)nitrate. The polyester and nitrate anions are removed by contacting the polymer membrane with an acidic solution.
A method for making a molecularly imprinted polymer membrane including a selective binding site for ferric cations according to an aspect of the invention is also provided. The method includes the steps of copolymerizing a solution comprising an matrix monomer, a cross-linking agent, an iron vinylbenzoate cation imprinting complex, a permeability agent, and an initiator for the polymerization mixture, so that a polymer membrane is formed, and removing ferric cations and the permeability agent from said polymer membrane so that cavities and channels for receiving ferric cations are formed in the membrane. The cavities are in communication with the exterior surface of the membrane by means of the channels. The matrix monomer is preferably styrene, the cross-linking agent is preferably divinylbenzene, the initiator is preferably azobisisobutyronitrile, and the permeability agent is preferably polyester. The iron vinylbenzoate cation imprinting complex is Fe(III)(vinylbenzoate)3. The polyester and ferric cations are removed by contacting the polymer membrane with an acidic solution.
The present invention also includes molecularly imprinted polymer membranes prepared by the inventive methods. According to another aspect of the invention, a molecularly imprinted polymer membrane is provided, and includes a selective binding site for phosphate anions characterized by channels disposed through said membrane to increase permeability and direct said phosphate anions to said binding sites. The membrane includes a cross-linked polymer matrix having channels disposed therethrough and with phosphate binding sites on the interior surface thereof in communication with the exterior surface of the membrane by means of the membrane channels. The membrane also has a thickness of approximately 100 microns.
According to another aspect of the invention, a molecularly imprinted polymer membrane is provided, and includes a selective binding site for nitrate anions characterized by channels disposed through said membrane to increase permeability and direct said nitrate anions to said binding sites. The membrane includes a cross-linked polymer matrix having channels disposed therethrough and with nitrate anion binding sites on the interior surface thereof in communication with the exterior surface of the membrane by means of the membrane channels. The membrane also has a thickness of approximately 100 microns.
According to another aspect of the invention, a molecularly imprinted polymer membrane is provided, and includes a selective binding site for ferric cations characterized by channels disposed through said membrane to increase permeability and direct said ferric cations to said binding sites. The membrane includes a cross-linked polymer matrix having channels disposed therethrough and with ferric cation binding sites on the interior surface thereof in communication with the exterior surface of the membrane by means of the membrane channels. The membrane also has a thickness of approximately 100 microns.
According to another aspect of the invention, the target ions are removed from an aqueous solution, collected, and reused in another application.